Testing of magnetic materials



June 21, 1955 R. o. ENDRES ET A1. 2,711,509

TESTING oF MAGNETIC MATERIALS Filed June 1, '1954 INI/ENTORS ATTORNEY 'finite States Patented .lune 21, 1955 TESTHQQ 0F MAGNETEC MATERIALS Richard Endres, Collingswood, and Lester L. Blair, Sonierdale, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application .lune 1, 1954, Serial No. 433,612

11 Claims. (Cl. 324-34) This invention relates to a magnetic materials testing apparatus, and particularly to improved apparatus for testing magnetic cores.

This invention is particularly designed for testing the type of cores that are used in a magnetic matrix memory system for computers, and in other related magnetic switching circuits, which required a multitude of magnetic cores of uniform magnetic properties. These cores are usually toroidal in shape with a relatively small inside diameter. The diameter of the hole in the core may be of the order of one-thirty second of an inch or less. The cores used at present are either wound out of very thin rolled metallic alloys on a ceramic bobbin or are molded ferrospinel types of material. In any case, the uniformity of magnetic properties depends a great deal on the care taken in the manufacturing techniques, which are quite critical.

The cores used in a magnetic matrix memory system are required to have a substantially rectangular hysteresis loop so that the residual magnetic induction after saturation and the magnetic induction present in a saturated condition are effectively the same. The polarity of the residual magnetism is used to represent the stored information. These systems are described by l. A. Rajchman in RCA Review, volume XIII, pages 1834201, June i952; and by W. N. Papian, Proceedings of the I. R. E, pages 475-478, April 1952.

All of the cores comprising a matrix may be coupled in rows and in columns of cores by individual single-turn coils through which information is stored on or read from the individual cores. This storage or read-out is accomplished by applying a current pulse to the coil coupling a particular row of cores and to the coil coupling a particular column of cores so that the single core lying in both that particular row and that particular column is energized by both current pulses. The pulses of current f supplied to each single-turn coil may be one half of the value required to set up a magnetomotive force sufficient to saturate the core. In this manner, the one-half H ampere turns (magnetomotive force) from each of the intersecting coils combine to provide a sufficient magnetomotive force to saturate the core in the direction desired. Because of the coil grid structure used, these one-half H pulses are also applied to the other cores in the row and column of the selected core. Therefore, there is a possibility that the magnetic polarity of these other cores may be disturbed if a driving pulse is in the opposite polarity to that of these other cores.

Thus, another requirement for a core may be that the coercive force of the core be more than one-half H. lf the coercive force were less than one-half H, a pulse of that value and of opposite magnetic polarity to that of the magnetized core would, because of the rectangular hysteresis loop, reverse the magnetism of the core and cause false information to be stored. Also, it may be required that the coercive force of a core be less than H, or else the current drives used, although adequate for some cores, would not be sufficient to change the magnetic polarity of the core with the excessive coercive force. Consequently, the cores used should have substantially uniform coercive forces falling between one-half H and H as extreme limits to allow a uniform drive to be used in the memory system.

lt is generally diliicult and expensive to control sufiiciently the manufacture of the cores to insure uniformity of the degree needed in the magnetic memory and switching systems. lt is simpler, more reliable and less expensive to screen-test the cores.

Gne of the preferred methods for testing magnetic materials is to determine its hysteresis characteristic curve. From this characteristic such magnetic material parameters as coercive force, remanent and saturated induction, and initial and maximum permeability are obtainable. Most arrangements in the prior art are adapted to test magnetic cores having a physical size that enables them to be magnetically coupled into the test apparatus with relative ease. The testing is usually accomplished by placing the core within primary and secondary coils, the variation in coupling caused by the cores serving to give the test data. Further, their large size makes it easy to use as many turns in the coupled coils as are needed. However, when the core to be tested is small, it becomes diflicult to use many turns, and, since the cross-sectional area is small, the resultant voltage is lower than is practical for test purposes.

Because of the coupling difficulties with small cores, on the order of one-sixteenth inch in overall diameter, it is quite diflicult to obtain a sufficient voltage to permit an observation of a hysteresis loop. it is not practical to use a high frequency sinusoidal magnetizing force drive upon the material being tested, to obtain more output voltage in a coupling secondary winding, because in some cases resulting increased eddy current losses would mask the desired information, and there are difficulties in getting faithful amplification at high frequencies.

In the copending patent applications by Rajchman, et al. Serial No. 346,892 filed March 25, 1953, and Serial No. 344,646 tiled March 25, 1953, test apparatus is described for supplying magnetizing current pulses to drive the cores to be te ed. The resulting voltages induced in a pickup coil linked to a test core contain the required test information. An oscilloscope may be employed for displaying the test information. The test apparatus described in the aforementioned patent applications employs a single pin that may be linked through the small hole in a core. The single pin provides a common primary and secondary winding for carrying the energizing currents and for forming part of a single-turn pickup coil.

This single-pin testing system has proven adequate for the testing of magnetic cores used in peak-toapeak or integration discrimination types of magnetic memories. Due to the integration discrimination techniques of memory interrogation, relatively high noise voltages of short duration have a relatively small effect on the overall signal-to-noise ratio. Therefore, in core testing, it is generally not necessary to measure very small noise voltage amplitudes. However, very small noise voltages are important in coincident-current magnetic memories employing a common sensing wire and time strobing read out techniques. in such time strobing techniques, the memory interrogation is performed by discriminating between core signal voltage and partial excitation noise volages at a particular instant when signal voltages are relatively high and noise voltages are relatively low. -lf the magnitude of the noise voltage from any one core is of the order of 200 microvolts, and signal voltage amplitudes are of the order of 80 millivolts, it would appear that the signal-to-noise ratio is very large. However, the combined action of many such cores in a large memory may result in accumulated noise voltage of the order of ten .E millivolts. Therefore, it is necessary in testing the magnetic cores to measure voltage amplitudes of the order of microvolts and eliminate cores having noise voltages in excess of the latter ligure.

The aforementioned single-pin core testing apparatus is not generally capable of reliably making such line measurements because the voltage drop caused by the primary pulse current flowing through the single pin may be several times the value of the induced voltage which must be measured. ln order to avoid such diiiiculties, separate primary and secondary windings may be soldered into place each time a core is to be tested. Such a procedure may be acceptable during occasional laboratory measurements. However, apparatus for rapidly checking many thousands of cores is needed.

Accordingly, it is among the obiects of this invention to provide:

l. New and improved apparatus for testing magnetic cores that are extremely small in size;

2. New and improved apparatus for testing extremely small magnetic cores and adapted for making very line voltage measurements;

3. New and improved apparatus for rapidly testing extremely small magnetic cores and adapted for reliably measuring very small voltages; and

4. A new and improved system for simply, rapidly and cheaply testing a large number of small magnetic cores for determining their magnetic properties.

In accordance with this invention, two slender pins are employed as separate primary and secondary windings linking the cores to be tested. The pins are made or electrically-conductive, non-magnetic material. Parallel end portions of the pins are held together as a unit with a thin non-conductive spacer element between them. The spacer element does not extend beyond the pins. Thus, a unitary structure is formed that is suiliciently small and free to be inserted in the holes of the test cores. The other ends of the pins are bent divergently and xedly attached to a mounting block. Connector terminals are attached to the fixed ends of the pins. Current pulses are applied to one of the pins through the fixed end connector terminal and a contact that removably engages the free end of that pin. A pickup coil is formed with the other pin through the fixed end connector terminal and another contact that removably engages the free end of the pickup coil pin. Test cores may be readily slipped over the free ends of the pins, and the contacts closed to make the desired electrical measurements.

The foregoing and other obiects, the advantages and novel features of this invention, as well as the invention itself both as to its organization and mode of operation, may be best understood from the following description when read together with the accompanying drawing, in which like reference numerals refer to like parts, and in which:

Figure l is a side view with parts enlarged of apparatus embodying this invention for testing magnetic cores and a block diagram of circuitry used in the apparatus;

Figure 2 is an enlarged sectional vicw along the line 2-2 of Figure 1;

Figure 3 is a top view of a detail of the apparatus along the line 3 3 of Figure 2; and

Figure 4 is an enlarged side View similar to Figure 2 and illustrating another embodiment of this invention.

Referring to Figures l, 2, and 3, a frame lil is providedl for supporting a portion of the test apparatus. The trame iti includes a base member l2 and a switching arm 14 pivoted 'for vertical movement on uprights 16 attached to one end of the hase member l2. The trame it? members are made up of insulating material, such as the phenol-aldehyde resin plastic Bakelite Upper and lower cylindrical mounting blocks l@ and .Ztl made of insulating material are respectively attached to the other ends of the switching arm i4 and the base member l2. The lower mounting block 2t) supports two test pins 22,

ett)

2din an upright position extending from the center or" the upper face of the block Ztl. rl`he upper mounting block lil supports two spaced sliding contacts 2.6, 28 extending rom and concentric with the lower face of the block E8. Concentric with each mounting block 1S, 2t) is an integral mounting peg 36, 32. The mounting pegs Si), 32 are held in aligned holes in the base member l?. and the switching arm ld so that the test pins 22, Z-iy and slifing contacts 26, 23 are also aligned.

The test pins 22, 24 are made of electrically conductive non-magnetic material such as silver. Parallel portions 3d of the pins 22, 2d are held together with a thin nonconductive spacer sheet such as a mica, between them. The mica sheet 36 is thinner than the pins 2.2, 24 and does not extend beyond the parallel portions 34 of the pins 22, 2d. The lower portions 3S ot the pins 22, 2d are bent divergently in a vertical plane. The lower mounting blocl' E@ has two cut-away portions on opposite sides forming vertical faces du. The bent ends 33 of the pins are attached to the vertical faces lil and also to solder lugs 42, 44S- held along the faces 4Q. inclined grooves 46 in the mounting block 2t? form a knife edge at the top surface oi the mounting block 2lb. The bent portions 3S of the pins are rmly held in the inclined grooves d6, and diverge down from the knife edge.

The parallel portions 34 of the pins and the mica sheet 36 may be glued together with a highly dielectric cement, such as a polyethylene plastic cement. ln addition, the pins 22, 24- and mica sheet 36 are held t0- gether and rigidly supported by a clamping ring 48 at the lower ends of the parallel pin portions 34. The upper ends of the pins 22, 2dare free. The clamping ri g 43 has a central hole in which the parallel pin portions 34 and the mica sheet 36 are tightly iitted. rl`he clamping ring 43 is made of non-conductive material, such as the polytetrailuoroethylene plastic Tetlonf and it is attached to the lower mounting block Zit. Cores Sil to be tested are threaded over the upper ends of the pins 22, 2.4 and are supported on the upper surface of the clamping ring 43 during testing.

Each of the sliding contacts Z6, 2S is a spring finger made of electrically conductive non-magnetic material such as a Phosphor bronze. This spring iingers 26, ZS are attached to cutout portions of the upper mounting block 18 by means of screws, and are normally spaced apart and aligned for individual engagement with the test pins 22, 24. Y

For testing cores Sil that have an internal diameter of .045 inch the following approximate dimensions are appropriate and have been successfully employed: The test pins 2?., 24 are each .020 inch in diameter, the mica sheet 36 is .005 inch thick, and the spring lingers 26, 28 are each .005 inch thick.

Current pulses are applied to the rst test pin 22 by means of a driving transformer S2. The transformer 52 has a pair of oppositely wound primary windings 5d and a single turn secondary winding 56. The secondary 56 is connected at one end through a lead 53 to the solder lug connector terminal Ll2 of the tirst pin 22, and, at the other end through a current regulating resistor oil and a lead 62 to the lirst spring contact 26. The transformer primary coils 54 are connected to a source 64 of current pulses. A suitable pulse source is described in the aforementioned patent application, Serial No. 346,892. A viewing oscilloscope 65 has a pair of input leads 65, 71B respectively connected to the solder lug connector terminal 44; of the second pin 2d and the second spring contact 28. The oscilloscope input leads 68, itl are twisted to balance out magnetic pick-up. lhe oscilloscope o5 also receives synchronizing pulses from the pulse source o4 to key the horizontal sweep circuit in conjunction with the test pulses.

Test cores Si? may be threaded over the free ends of the test pins 22, 24 by means ot a pointed permanent magnet. (not shown). The magnetic cores Si? tend to stand with their axis perpendicular to the axis of the pointed magnet. Consequently, it is easy to slip the core Si? over the test pins 22, 24 and then remove the magnet leaving the core on the pins 22, 24. The switching arm 14 is moved down to engage the sliding spring contacts 26, 28 with the associated test pins 22, 24. As a result, the energizing current circuit from the transformer secondary 56 is completed through the tirst test pin 22 and the iirst sliding contact 26, and a voltage pick-up coil is completed through the second test pin 24 and the second sliding contact 28 to the oscilloscope 66. Although extremely small dimensions are involved, the energizing and voltage pickup circuits through the two pins are independent. Therefore, extremely accurate and fine measurements can be made.

The driving transformer 52 produces the high current required for the single energizing winding through the test core 5t) that is provided by the first test pin 22. By means of the arrangement of two opposing primary windings 54, opposing polarities of current flow may be produced in the transformer secondary S6. The transformer core 52 should be large so that it does not saturate. A relatively large resistance 60 is connected in series with the first test pin 22 to insure a current drive independent of the load provided by the test core St). The actual energizing current flowing through the iirst pin 22 may be measured by the voltage across this series resistance 60. Appropriate test circuitry and measuring procedures are described in the aforementioned patent application, Serial No. 346,892.

Another embodiment of this invention is shown in Figure 4 in which parts already described have the same reference numerals. The cylindrical lower mounting 1block 72 has two oblique holes 74, 76 drilled from the lower to the upper surface converging at a knife edge at the upper surface. These holes 74, 76 receive the bent portions 38 of the test pins 22, 24. Horizontal holes 78, Sti from the sides of the mounting block 72 intersect the oblique holes 74, 76. Separate brass connector pins 82, 84, inserted in the horizontal holes 78, 2t?, frictionally engage the bent portions 38 of the test pins 22, 24. Thus, the test pins 22, 24 are firmly attached to the lower mounting block 72, and in electrical contact with the connector pins 80, 82. The outer heads of the connector pins 80, 82 may be used as connector terminals. In this way, a simple construction and fast assembly of apparatus is made possible. The construction and operation of the embodiment shown in Figure 4 is otherwise the same as that described above.

lt is seen from the above description of this invention that an improved apparatus is provided for testing magnetic cores that are extremely small in size. This improved apparatus is adapted for reliably measuring very small voltages, and for simply, rapidly and cheaply testing a large number of cores.

What is claimed is:

1. A device for testing the characteristics of magnetic cores comprising two electrically-conductive non-magnetic pins to which the cores to be tested may be linked, said pins having substantially parallel portions and divergent portions, a non-conductive spacer element fixed to said pins between said parallel portions, a mounting evice, means for securing said divergent portions to said mounting device, a path for supplying current to one of said pins connected to the divergent portion thereof and including first contact means for connecting said path to said one pin parallel portion, pickup coil means connected to the divergent portion of said other pin and including second contact means for engaging said other Cir pin parallel portion, and means for completing said current path and said pickup coil through said one and said other pins respectively including means for respectively engaging said iirst and second contact means with said one and said other pin parallel portions.

2. A testing device as recited in claim l wherein the breadth of said non-conductive spacer element is substantially the same as the breadth of said pins.

3. A testing device as recited in claim 2 wherein the length of said non-conductive spacer element is sub stantially the same as the length of said pin parallel por tions.

4. A testing device as recited in claim 2 wherein the thickness of said spacer element is less than the thickness of said pin parallel portions.

5. A testing device as recited in claim 1 wherein said mounting device includes means for supporting cores to be tested at said parallel pin portions.

6. A testing device as recited in claim 5 wherein said means for supporting cores to be tested includes a ring enclosing said pins along parts of the parallel portions thereof.

7. Apparatus for testing the characteristics of rnagnetic cores having a hole therethrough comprising two electrically-conductive, nonemagnetic pins to which the cores to be tested may be linked, a mounting device, means for fixing one end portion of each said pins to said mounting device, said pins having substantially parallel free end portions for receiving cores to be tested, a thin non-conductive spacer element lixed to said pins between said free end portions, said spacer elcment and said free end portions having substantially the same breadth, said mounting device including a clamping ring enclosing said pin parallel portions adjacent the fixed end of said pin, means for alternatively applying current pulses to one of said pins in opposite directions therethrough, said pulse applying means being connected to the fixed end portion of said one pin and including lirst contact means for removably engaging the free end portion of said one pin, and single-turn open pickup coil means connected to the fixed end portion of the other of said pins and including second contact means for removably engaging the free end portion of said other pin.

8. Testing apparatus as recited in claim 7 wherein said spacer element extends substantially the length of said parallel pin portions, wherein said mounting device further includes a mounting block to which said fixed end portions of said pins and clamping ring including means for supporting cores to be tested at said parallel pin portions, and wherein said first and second contact means includes resilient sliding contact elements.

9. Testing apparatus as recited in claim 8 wherein said fixed end portions of said pins are divergent.

10. Testing apparatus as recited in claim 9 wherein said pulse applying means and said pickup coil means include separate terminal lugs respectively fixed to the divergent portions of said one and said other pins at said mounting block.

1l. Testing apparatus as recited in claim 10 wherein said pulse applying means and said pickup coil means include separate terminal elements respectively pinned to the divergent portions of said one and said other pins at said mounting block.

Crouch Jan. 26, 1926 Rajchman et al May 18, 1954 

